1. Field of the Invention
The present invention relates to a method of controlling cell loss rates for data transmission in ATM (asynchronous transfer mode) network.
2. Description of the Background Art
In the ATM network, the information generated at each terminal is transmitted in units of fixed length packets called cells. In transmitting the cells, the request connections from the terminals are admitted to the network by such a number that an arithmetic sum of the maximum cell transmission speeds of the terminals can be greater than the bandwidth of the physical transmission lines, by taking the statistical multiplexing effect into account. Thus, the arithmetic sum of the cell transmission speeds of the terminals exceeds the bandwidth of the physical transmission lines at some probability, and therefore there is a possibility for the cells transmitted from the terminals to be discarded at some probability. Namely, in the ATM network, up to a certain level of overloading of the cells can be handled without discarding any cell at all by utilizing the buffering function of the network, but when the cells to be transmitted exceeds the buffer capacity, the buffer overflow occurs and some cells are discarded.
Now, in the ATM network, the management of he bandwidth for transmitting the cells is carried out by taking the statistical multiplexing effect into account. Namely, when each terminal makes a connection set up request to the network, the terminal also makes a request for a quality of service concerning the cell loss rate at least, and the network in response carries out the so called connection admission control in which the admission or rejection of the requested connection set up is determined according to a judgement as to whether it is possible to satisfy the requested quality of service by admitting this connection set up. In order to carry out this connection admission control accurately, it is necessary to make an estimation for the cell loss rate as accurately as possible for the sake of achieving more efficient network performance, and to be an over-estimation for the sake of securing the safety in the network operation. In other words, in the ATM network, the communication resource (bandwidth) is shared by a plurality of virtual channels by allowing the cell loss up to a certain cell loss rate, in order to improve the efficiency of the network performance.
Moreover, in the ATM network, it is possible for the network to distinguish those cells for which the cell loss is not tolerable at all and those cells for which the cell loss up to a certain level of the cell loss rate is tolerable, by specifying an explicit indication of the priority level regarding the cell loss in the CLP (cell loss priority) field provided in the ATM header of the cell to be transmitted from the terminal to the network. In this case, when the need for discarding some cells arises because of congestion, the network starts discarding the cells with the lower priority level first. Thus, in this case, two types of cells with two different cell loss priority levels are mixed within a single virtual channel, and the network needs to carry out the cell loss priority control for these two types of cells, along with the usual connection admission control for securing the quality of service.
However, a method of quality of service control regarding the cell loss for a case in which a plurality of priority levels are involved within a single virtual channel has not been established yet.
Moreover, in the ATM network, the following two conditions must be satisfied in a case of using the CLP field in the ATM header.
(1) The quality of service regarding the cell loss for each virtual channel requested by the terminal must be secured.
(2) In a case of discarding cells, the network must secure that it starts discarding the cells with the lower priority level (loss tolerable cells) first, before discarding any cell with the higher priority level (loss intolerable cell).
The condition (2) is required obviously because otherwise the priority level specified in each cell loses any significance at all.
As for the condition (1), when two types of loss tolerable cells and loss intolerable cells are present among the cells of a single virtual channel and these two types of cells are controlled according to the condition (2), it is inevitable that the actual cell loss rate for the loss tolerable cells and the actual cell loss rate for the loss intolerable cells turn out to be different even within a single virtual channel. Thus, it is necessary to establish exactly what should be secured by the network and what should be requested by the terminal as the quality of service concerning the cell loss for each virtual channel, and to devise an algorithm for the connection admission control capable of securing the quality of service regarding the cell loss.
On the other hand, it is also necessary for the network to monitor whether the traffic characteristics declared by each terminal at a time of the connection set up request is faithfully maintained or not, and to regulate the cell flow in a case there is a violation. This operation of the network is called a usage parameter control. Here, it is necessary to establish how this usage parameter control should be carried out with respect to the two types of the cells with two different cell loss priority levels.
Furthermore, when the a plurality of cell groups (virtual channels) containing the loss tolerable cells and loss intolerable cells at different ratio share the same communication resource, the network does not recognize the individual virtual channel, so that in discarding the cells, all the loss tolerable cells are discarded uniformly, regardless of which virtual channel they belong to. As a consequence, the virtual channels containing more loss tolerable cells may end up losing more cells, such that the total cell loss rate becomes much larger for these virtual channels than for the other virtual channels. In other words, in such a case, it is impossible to secure the total cell loss rate for each virtual channel.